Injection moulding may be used to make a wide variety of articles including articles having relatively complex shapes and a range of sizes. A particularly important application is in the manufacture of caps and closures for food and drink applications, such as for bottles containing carbonated or non-carbonated drinks, or for non-food applications such as containers for cosmetics or pharmaceuticals.
Important properties of the polymer to be injection moulded are its rheology, stiffness, environmental stress crack resistance (ESCR) and organoleptic properties, requirements for all of which need to be satisfied at the same time. Injection moulded articles, particularly caps and closures, may be in contact with aggressive food and non-food media and/or subject to external stress, e.g. when used as a cap for a carbonated soft drink, and a high ESCR is therefore desirable. High ESCR values are generally obtained with polyethylenes of lower density and/or lower melt index. On the other hand, injection moulded articles also require a high stiffness as this allows a reduction in wall thickness while maintaining good dimensional stability. Stiffness generally increases as density increases.
It is further important in injection moulding that the polymer melt has the appropriate rheological properties, i.e. a flowability within certain limits to ensure that the final product properties are desirable. For example, the flowability of the polymer melt must be sufficiently high to enable it to flow to all areas of the mould when injected so as to form the desired article. Also, the higher the flow rate of the polymer melt, the greater the speed at which it can be injected into the mould and the shorter the processing time, which improves productivity. A measure of flowability which is particularly relevant to injection moulding is spiral flow, which measures the length around a spiral which molten polymer flows under a particular pressure, temperature and injection rate. A higher spiral flow indicates better processability.
For improving the flow properties, polyethylenes with broader molecular weight distribution or with higher melt index are typically chosen. However, polymers with higher melt index tend to yield products having poor ESCR. Also, a polymer with a broad molecular weight distribution may also lead to a larger degree of orientation of the polymer chains in the finished injection moulded article, which may contribute to the aforementioned poorer mechanical properties. Polyethylenes having a narrow molecular weight distribution and a low melt index are therefore better suited to reaching the desired environmental stress crack resistance, at the expense of good flow properties.
For improving the stiffness of the final article such as a cap, it is well known to increase the density of the polymer. However a higher density tend to result in a poorer ESCR.
Furthermore, especially with regard to the food applications of caps and closures, it is important that the composition has good taste and odour properties and low levels of extractables that can migrate into the food. A narrower molecular weight distribution is preferred as it means a smaller proportion of very low molecular weight material in the composition, resulting in fewer volatile fractions which could migrate into food.
Our own EP 1441959A exemplifies bimodal polyethylene compositions for caps and closures having densities in the range 951-953 kg/m3, MI2 values in the range 0.6-1.7 g/10 min, LMW:HMW component ratios of approximately 50:50. Values of ESCR-B above 1000 h are reported as measured on compression moulded specimen, but no ESCR data obtained on injected caps is reported. No SHI1/100 or spiral flow values are disclosed.
WO 2007/130515 discloses polyethylene compositions suitable for caps and closures having a density of 950-960 kg/m3 and a MI2 of preferably 1-2 g/10 min and a g′>1.
EP 1655338A discloses a polyethylene composition for caps and closures which has an MI2 of 0.1 to 100 g/10 min, a Charpy impact strength at 23° C. of at least 3 kJ/m2 and a specific relationship between SHI1/100 and log MI2. All the inventive Examples are multimodal, with a density of at least 961 kg/m3 and a SHI1/100 between 8 and 14.
EP 1655336A also discloses a polyethylene composition for caps and closures, this one having an MI2 of 0.1 to 100 g/10 min, an ESCR of at least 10 hours and a different relationship between SHI1/100 and log MI2. All the inventive Examples are multimodal, with a density between 956 kg/m3 and 961 kg/m3 and a SHI1/100 of between 14 and 22.
EP2017302A discloses a hexene copolymer for caps and closures which has an MI2 of 0.1 to 100 g/10 min, a tensile modulus >850 MPa, an ESCR-B of at least 300 hours and a Charpy impact strength at 23° C. of at least 15 kJ/m2. All the examples have a molecular weight distribution (Mw/Mn) between 15 and 22.
WO2011004032 discloses a bimodal polyethylene composition for caps and closures made with a metallocene catalyst comprising two polyethylene fractions A and B, fraction A being substantially free of comonomer and providing an improved balance between environmental stress crack resistance, organoleptic properties, dimensional stability, tightness of fit and ease of opening. All inventive examples are characterised by a narrow molecular weight distribution (Mw/Mn<5).
WO2007018720 discloses a bimodal polyethylene composition for injection moulding comprising two polyethylene fractions A and B which is preferably made with a metallocene catalyst. The preferred melt index of the blend is at least 4 g/10 min, and the Examples combine an overall melt index of above 4 g/10 min with a HMW fraction having a melt index of at least 0.46 g/10 min, which equates to an HLMI of above 10 g/10 min. A HMW fraction having a high melt index/low molecular weight generally results in a composition having relatively poor stress crack resistance.